1. Field of the Invention
The present invention relates to a method of transporting a photosensitive sheet in an image forming apparatus, and more particularly relates to a method of successively transporting a photosensitive sheet from a roll of the sheet wound up around a sheet-feeding rod, and then through image forming process units such as an exposure section and a pressure-developing section, and thereafter onto a take up rod.
2. Description of the Prior Art
FIG. 5 diagrammatically shows the main part of a typical image forming apparatus which utilizes a photosensitive sheet 51 and an image receiving sheet 55. One side of the photosensitive sheet 51 is coated with pressure-rupturable microcapsules containing a light sensitive hardener and a chromogenic material. Because of the light sensitive hardener, the microcapsules harden when exposed to light. One side of the image receiving sheet is coated with a developing material which reacts with the chromogenic material to give rise to color in the chromogenic material. The photosensitive sheet 51 is previously rolled up around a sheet-feeding rod 52. The roll of the photosensitive sheet 51 is disposed within a cartridge 53. The cartridge 53 protects the photosensitive sheet 51 from undesirable light or pressure. The free end of the photosensitive sheet 51 is drawn out of the cartridge 53, and transported through image forming process units such as an exposure section P1, a pressure-developing section P2, and the like, and then wound up around a take up rod 54. In this state, image forming operation starts, as will be described below.
First, an exposure process is performed as follows: A predetermined length of the photosensitive sheet 51 is drawn out from the sheet-feeding rod 52 to be transported through the exposure section P1, where it is exposed to light reflected from an original to be copied. The light selectively illuminates the photosensitive sheet 51 so that some of the microcapsules thereon receive light to harden and the other microcapsules do not receive light and are left unchanged, thereby forming a latent image corresponding to the original on the photosensitive sheet 51. The front end of the thus produced latent image is located in a position on the photosensitive sheet 51 which was located at the exposure section P1 before the exposure process.
After the exposure process, the portion of the photosensitive sheet 51 having the latent image thereon is conveyed toward the pressure-developing section P2. When the front end of the latent image reaches the pressure-developing section P2, the image receiving sheet 55 is also fed thereinto, thereby starting a pressure-developing process. In the pressure-developing process, the photosensitive sheet 51 and the image receiving sheet 55 are pressed against each other while they are passing through the pressure-developing section P2. As a result, the microcapsules which have not hardened rupture, allowing the chromogenic material therein to flow out onto the image receiving sheet 55. The chromogenic material then reacts with the developing material on the image receiving sheet 55 to impart color thereto, resulting in a visible image on the image receiving sheet 55. This pressure-developing process is completed when the rear ends of both the latent image and the image receiving sheet 55 pass through the pressure-developing section P2. After the pressure-developing process, the photosensitive sheet 51 is further transported through the pressure-developing section P2 toward the take up rod 54 until the image receiving sheet 55 with the visible image formed thereon is separated from the photosensitive sheet 51 at a separating roller 56.
At the end of the image forming operation described above, the rear end of the latent image on the photosensitive sheet 51 is located ahead of the pressure-developing section P2, while a certain length of the photosensitive sheet 51 which has not been used for image formation remains between the exposure section P1 and the pressure-developing section P2. If the next exposure process begins immediately after the above image forming operation, the front end of the next latent image to be formed will be in a position on the photosensitive sheet 51 which is now located at the exposure section P1. Thus, the above-mentioned portion of the photosensitive sheet 51 located between the exposure section P1 and the pressure-developing section P2 will remain unused and wasted.
A method of transporting a photosensitive sheet has been proposed in which the above-mentioned unused portion of the photosensitive sheet is transported backward to be used for the formation of the next latent image. In this conventional method, however, the actual length of the unused portion of the photosensitive sheet which has been transported in the preceding image forming operation is not measured. The photosensitive sheet is just transported backward for a predetermined period of time after each image forming operation. Thus, it is difficult to accurately transport the photosensitive sheet backward by the actual length of the unused portion.
The inaccuracy in the backward transportation of the photosensitive sheet causes the following problems. If the photosensitive sheet is transported backward by a length shorter than that of the unused portion of the sheet, part of the unused portion remains between the exposure section and the pressure-developing section and is accordingly wasted. If the photosensitive sheet is transported backward by a length longer than that of the unused portion of the sheet, the portion of the sheet which has passed through the pressure-developing section is also transported backward to be used for the next image formation. As described above, this portion of the photosensitive sheet has been transported through the pressure-developing section to allow the image receiving sheet to be separated from the photosensitive sheet at the separating roller. Since this portion may have been scratched while passing through the pressure-developing section, the latent image to be formed on this portion in the next image forming operation will have poor quality.